Process of regenerating fused saltpeter baths



Patented Apr. 1940 UNITED STATES PROCESS or nnenmznarmo rusnn sanrrn'mn BATES Josef Martin Michel, Bitterield, Germany, as-. signor to I. G. Farbenind-trie Aktiengeselh. schaft, Frankfort-on-the-llaln, Germany No Drawing. Application April 12, 1938, Serial No. 201,639. In Germany April 27, 1937 2 Claims. (01. 252-6) This invention relates to a process of regenerating fused saltpeter baths which have, in the course of their employment, become partially decomposed.

Fused alkali metal nitrates (i. e. potassium or sodium nitrate) or mixtures thereof are frequently employed in the heat-treatment of alloys, particularly aluminium alloys, as a means for heating the metal to or maintaining it at certain temperatures in a reliable and easily controllable manner. It has now however been ascertained that a partial progressive decomposition of such fused nitrate baths occurs in the course of usage, such decomposition at first only yielding nitrite. and oxygen; the nitrite formed then gradually further decomposes so as to ultimately yield free alkali. The free oxygen formed by decomposition is apt to react with the surfaces of the metal to be treated as well as with any exposed metallic surface of the containers of the baths, whilst the free alkali formed by further decomposition, in turn. is capable of reacting with such metal oxides, with formation, e. g. of alkaline reacting aluminates and ferrites,

" which become dissolved in the saline melt. This attack of the decomposition products present in the degenerated saline baths of the kind referred to, upon exposed metallic surfaces, is particularly strong in the temperature range at which the baths are generally employed (about 200-500 C.) w

The present invention aims at regenerating such degenerated fused nitrate baths. This is achieved, according to the invention, by adding to the salt melts such amounts of alkali metal dichromate as are sufllcient to convert the alkali metal oxides or other alkaline reaction compounds resulting from the decomposition, and also any alkali metal nitrite formed in the first stage of decomposition into monochromate, for example in accordance with the equations:

1 to 3 have been completed and only becomes complete after a few days, when all the nitrite originally present in the "degenerated bath has become decomposed.

. The presence of the monochromate thus formed in the bath substantially prevents the further decomposition of the latter owing to the existence of equilibria between alkali metal nitrate, alkali metal nitrite and. alkali metal monochromate, in which the proportion of alkali metal nitrite is extremely small.

The required amount of alkali metal dichromate (to be calculated on the basis of the foregoing Equations lto 4) may either be added all at once to the "degenerated melt, or else the amounts determined fromlfiquations l to 3 may first be added and, after neutralisation has taken place. the portions required for the reaction with the nitrite present are added; 7

The regeneration of the bath by addition of bichromate is preferably effected at temperatures between about 300 and 350 .C., since at higher temperatures the reactions occurring as aresult of the addition of bichromate may assume an explosive character.

Example To a degenerated saltpeter melt originally consisting of a mixture of equal parts of potassium nitrate and sodium nitrate andcontaining, as the products of decomposition of the original bath, according to analysis, 1.9% by weight of nitrite' (calculated as N203) in addition of 0.3% by weight of alkaline reacting components (calculated as 2 with formation of monochromate, the alkalinity of the bath, was first added. After two hours the 40 melt, according to a test made, had become completely neutralised. Thereupon, the amount of dichromate calculated as being necessary to de-- compose the alkali metal nitrite, was added. In the course of a few days the N20: value of the bath was then observed to vary as follows:

Percent 01 mos.

After four days the melt contained practically no more nitrite and was thus completelyreaenerated.

I claim:-

1. A process 01' regenerating fused saltpeter baths, tor the heat-treatment of alloys, which have degenerated in the course of use with the formation of decomposition products comprising free alkali, alkali metal aluininates or ferrites, alkali metal nitrite, which comprises adding to such iused anhydrous baths at a temperature not substantially exceeding about 350 C. an alkali metal dichromate in the amount required to convex-t such decomposition products into monochromate.

2. Process in accordance with claim 1, in which at first only such amounts of dichromate are added to the baths as are necessary to neutralise the alkalinity of the baths, and after neutralisation has taken place, a further quantity of dichromate sumcient to decompose the nitrite present, with formation of monochromate, is added.

JOSEF MAR'I'IN MICHEL. 

